Preheat the oven to 210°C (415°F/Gas 6–7). Brush a 20cm (8inch) square cake tin with oil or melted butter. Line the base with baking paper.
Put the polenta and sugar in a large bowl. Add the sifted flour, baking powder, bicarbonate of soda and ½teaspoon salt and mix thoroughly.
In a separate bowl, combine the egg, buttermilk and butter. Stir the mixture quickly into the dry ingredients, stirring only until the ingredients are moistened. Pour the mixture into the prepared tin and smooth the surface. Bake for 20–25 minutes, or until a skewer inserted in the centre of the bread comes out clean.
Place on a wire rack and leave for 10 minutes to cool before turning out. Cut into squares and serve warm.